T-Mobile tries to lure away iPhone customers from AT&T offering savings of $50/month compared to AT&T. You need though an unlocked iPhone 4S or iPhone 5.
T-Mobile USA does not have the iPhone. It is the biggest problem the carrier has. On September 12 they will launch an aggressive campaign to try to get iPhone customers from AT&T. The T-Mobile Bring your own iPhone effort tries to convince owners of unlocked iPhones that T-Mobile USA has faster service at a lower price.
T-Mobile USA says that you save $50 per month based on comparison of T-Mobile $69.99/mo Unlimited Talk, Text and Data plan against AT&T Unlimited Talk, Text, and 5GB Smartphone Data plan. The comparison does not include taxes, fees, or promotional offers. Plan features and limitations vary.
T-Mobile also says that in their internal tests of unlocked iPhone 4S devices running over 4G (HSPA+) on T-Mobile 1900 network recorded on average 70% faster download speeds than iPhone 4S devices on AT&T’s network.